The 20th century was the period between January 1, 1901[1] and December 31, 2000,[2][3] inclusive. It was the tenth and last century of the 2nd millennium. It is distinct from the century known as the 1900s (sometimes written 19XX), which began on January 1, 1900 and ended December 31, 1999.

The century had the first global-scale wars between world powers across continents in World War I and World War II. Nationalism became a major political issue in the world in the 20th century, acknowledged in international law along with the right of nations to self-determination, official decolonization in the mid-century, and nationalist-influenced armed conflicts – including two World Wars.

The century saw a major shift in the way that many people lived, with changes in politics, ideology, economics, society, culture, science, technology, and medicine. The 20th century may have seen more technological and scientific progress than all the other centuries combined since the dawn of civilization. Terms like ideology, world war, genocide, and nuclear war entered common usage. Scientific discoveries, such as the theory of relativity and quantum physics, profoundly changed the foundational models of physical science, forcing scientists to realize that the universe was more complex than previously believed, and dashing the hopes (or fears) at the end of the 19th century that the last few details of scientific knowledge were about to be filled in. It was a century that started with horses, simple automobiles, and freighters but ended with high-speed rail, cruise ships, global commercial air travel and the space shuttle. Horses, Western society's basic form of personal transportation for thousands of years, were replaced by automobiles and buses within a few decades. These developments were made possible by the exploitation of fossil fuel resources, which offered energy in an easily portable form, but also caused concern about pollution and long-term impact on the environment. Humans explored space for the first time, taking their first footsteps on the Moon.

Mass media, telecommunications, and information technology (especially computers, paperback books, public education, and the Internet) made the world's knowledge more widely available. Advancements in medical technology also improved the welfare of many people: the global life expectancy increased from 35 years to 65 years. Rapid technological advancements, however, also allowed warfare to reach unprecedented levels of destruction. World War II alone killed over 60 million people, while nuclear weapons gave humankind the means to annihilate itself in a short time. However, these same wars resulted in the destruction of the Imperial system. For the first time in human history, empires and their wars of expansion and colonization ceased to be a factor in global affairs and left an increasingly globalized and cooperative world in their wake. The last time major powers clashed openly was 1945, and since then, violence has seen an unprecedented decline.[4]

The world also became more culturally homogenized than ever with developments in transportation and communications technology, popular music and other influences of Western culture, international corporations, and what was arguably a true global economy by the end of the 20th century.

Technological advancements during World War I changed the way war was fought, as new inventions such as tanks, chemical weapons, and aircraft modified tactics and strategy. After more than four years of trench warfare in western Europe, and 20 million dead, the powers that had formed the Triple Entente (France, Britain, and Russia, later replaced by the United States and joined by Italy) emerged victorious over the Central Powers (Germany, Austria-Hungary, and the Ottoman Empire). In addition to annexing much of the colonial possessions of the vanquished states, the Triple Entente exacted punitive restitution payments from them, plunging Germany in particular into economic depression. The regime of Tsar Nicholas II was overthrown during the conflict, Russia became the first communist state, and the Austro-Hungarian and Ottoman empires were dismantled at the war's conclusion.

At the beginning of the period, Britain was the world's most powerful nation,[6] having acted as the world's policeman for the past century. Fascism, a movement which grew out of post-war angst and which accelerated during the Great Depression of the 1930s, gained momentum in Italy, Germany and Spain in the 1920s and 1930s, culminating in World War II (1939–1945), sparked by Nazi Germany's aggressive expansion at the expense of its neighbors. Meanwhile, Japan had rapidly transformed itself into a technologically advanced industrial power. Its military expansion into eastern Asia and the Pacific Ocean culminated in a surprise attack on the United States, bringing it into World War II. After some years of dramatic military success, Germany was defeated in 1945, having been invaded by the Soviet Union from the east and by the United States, the United Kingdom, Canada, and Free France from the west. The war ended with the dropping of two atomic bombs on Japan. Japan later became a western ally with an economy based on the manufacture of consumer goods and trade. Germany was divided between the Western powers and the Soviet Union; all areas recaptured by the Soviet Union (East Germany and eastward) became Soviet puppet states under communist rule. Meanwhile, Western European countries were influenced by the American Marshall Plan and made a quick economic recovery, becoming major allies of the United States under capitalist economies and relatively democratic governments.

World War II left about 60 million people dead. When the conflict ended in 1945, the United States and the Soviet Union emerged as the major world powers. Allies during the war, they soon became hostile to one another as the competing ideologies of communism and democratic capitalism occupied Europe, divided by the Iron Curtain and the Berlin Wall. The military alliances headed by these nations (NATO in North America and Western Europe; the Warsaw Pact in Eastern Europe) threatened each other with total war in what was called the Cold War (1947–91). The period was marked by a new arms race, and nuclear weapons were produced in the tens of thousands, sufficient to end most human life on the planet had a large-scale nuclear exchange ever occurred. The size of the nuclear arsenals is believed by many historians to have staved off war between the two, as the consequences were too great to bear. The policy of massive nuclear attack, knowing a similar counterattack would be forthcoming, was called mutually assured destruction (MAD). However, several proxy wars, such as the Korean War (1950–1953) and the Vietnam War (1957–1975), were waged as the United States implemented its worldwide "containment" policy against communism.

After World War II, most of the European-colonized world in Africa and Asia gained independence in a process of decolonization. Meanwhile, the wars empowered several nations, including the UK, USA, Russia, China and Japan, to exert a strong influence over many world affairs. American culture spread around the world with the advent of Hollywood, Broadway, rock and roll, pop music, fast food, big-box stores, and the hip-hop lifestyle. Britain continued to influence world culture, including the "British Invasion" into American music, leading many rock bands from other countries (such as Swedish ABBA) to sing in English. The western world and parts of Asia enjoyed a post-World War II economic boom. After the Soviet Union collapsed under internal pressure in 1991, the communist governments of the Eastern bloc were dismantled, followed by awkward transitions into market economies.

Following World War II the United Nations, successor to the League of Nations, was established as an international forum in which the world's nations could discuss issues diplomatically. It enacted resolutions on such topics as the conduct of warfare, environmental protection, international sovereignty, and human rights. Peacekeeping forces consisting of troops provided by various countries, with various United Nations and other aid agencies, helped to relieve famine, disease, and poverty, and to suppress some local armed conflicts. Europe slowly united, economically and, in some ways, politically, to form the European Union, which consisted of 15 European countries by the end of the 20th century.

In the last third of the century, concern about humankind's impact on the Earth's environment made environmentalism popular. In many countries, especially in Europe, the movement was channeled into politics through Green parties. Increasing awareness of global warming began in the 1980s, commencing decades of social and political debate.

Due to continuing industrialization and expanding trade, many significant changes of the century were, directly or indirectly, economic and technological in nature. Inventions such as the light bulb, the automobile, and the telephone in the late 19th century, followed by supertankers, airliners, motorways, radio, television, antibiotics, frozen food, computers and microcomputers, the Internet, and mobile telephones affected people's quality of life across the developed world. Scientific research, engineering professionalization and technological development drove changes in everyday life.

At the beginning of the century, strong discrimination based on race and sex was significant in general society. Although the Atlantic slave trade had ended in the 19th century, the fight for equality for darker-skinned people in the white societies of North America, Europe, and South Africa continued. During the century, the social taboo of sexism fell. By the end of the 20th century, women had the same legal rights as men in many parts of the world, and racism had come to be seen as abhorrent.[7]

The number of people killed during the century by government actions was in the hundreds of millions. This includes deaths caused by wars, genocide, politicide and mass murders. The deaths from acts of war during the two world wars alone have been estimated at between 50 and 80 million[citation needed]. Political scientist Rudolph Rummel estimated 262,000,000 deaths caused by democide, which excludes those killed in war battles, civilians unintentionally killed in war and killings of rioting mobs.[8] According to Charles Tilly, "Altogether, about 100 million people died as a direct result of action by organized military units backed by one government or another over the course of the century. Most likely a comparable number of civilians died of war-induced disease and other indirect effects."[9] It is estimated that approximately 70 million Europeans died through war, violence and famine between 1914 and 1945.[10]

After gaining political rights in the United States and much of Europe in the first part of the century, and with the advent of new birth control techniques, women became more independent throughout the century.

Rising nationalism and increasing national awareness were among the many causes of World War I (1914–1918), the first of two wars to involve many major world powers including Germany, France, Italy, Japan, Russia/USSR, the United States and the British Empire. World War I led to the creation of many new countries, especially in Eastern Europe. At the time, it was said by many to be the "war to end war".

Civil wars occurred in many nations. A violent civil war broke out in Spain in 1936 when General Francisco Franco rebelled against the Second Spanish Republic. Many[who?] consider this war as a testing battleground for World War II, as the fascist armies bombed some Spanish territories.

The Cold War had caused an arms race and increasing competition between the two major players in the world: the Soviet Union and USA. This competition included the development and improvement of nuclear weapons and the Space Race.

The Soviet authorities caused the deaths of millions of their own citizens in order to eliminate domestic opposition.[11] More than 18 million people passed through the Gulag, with a further 6 million being exiled to remote areas of the Soviet Union.[12]

The two world wars led to efforts to increase international cooperation, notably through the founding of the League of Nations after World War I, and its successor, the United Nations, after World War II.

Nationalist movements in the subcontinent led to the independence of India and Pakistan.

Gandhi's non-violence and civil disobedience influenced many political movements around the world, including the African American civil rights movement in USA, and freedom movements in South Africa and Burma.

The creation in 1948 of Israel, a Jewish state in the Middle East, by the British Mandate of Palestine fueled many regional conflicts. These were also influenced by the vast oil fields in many of the other countries of the mostly Arab region.

The end ofcolonialism led to the independence of many African and Asian countries. During the Cold War, many of these aligned with the United States, the USSR, or China for defense.

After a long period of civil wars and conflicts with European powers, China's last imperial dynasty ended in 1912. The resulting republic was replaced, after another civil war, by a communist People's Republic in 1949. At the end of the 20th century, though still ruled by a communist party, China's economic system had largely transformed to capitalism.

The Tiananmen Square protests of 1989, culminating in the deaths of hundreds of civilian protesters, were a series of demonstrations in and near Tiananmen Square in Beijing, China. Led mainly by students and intellectuals, the protests occurred in a year that saw the collapse of a number of communist governments around the world.

As the century began, Paris was the artistic capital of the world, where both French and foreign writers, composers and visual artists gathered.

Movies, music and the media had a major influence on fashion and trends in all aspects of life. As many movies and much music originate from the United States, American culture spread rapidly over the world.

Tango was created in Argentina and became extremely popular in the rest of America and Europe. Blues and jazz music became popularized during the 1910s and 1920s in the United States. Blues went on to influence rock and roll in the 1950s, which only increased in popularity with the British Invasion of the mid-to-late 1960s. Rock soon branched into many different genres, including heavy metal, punk rock, and alternative rock and became the dominant genre of popular music. This was challenged with the rise of hip hop in the 1980s and 1990s. Other genres such as house, techno, reggae, and soul all developed during the latter half of the century and went through various periods of popularity.

The modern art movement revolutionized art and culture and set the stage for both Modernism and its counterpart postmodern art as well as other contemporary art practices.

Art Nouveau began as advanced architecture and design but fell out of fashion after World War I. The style was dynamic and inventive but unsuited to the depression of the Great War.

In Europe, modern architecture departed from the decorated styles of the Victorian era. Streamlined forms inspired by machines became commonplace, enabled by developments in building materials and technologies. Before World War II, many European architects moved to the United States, where modern architecture continued to develop.

The automobile increased the mobility of people in the Western countries in the early-to-mid-century, and in many other places by the end of the 20th century. City design throughout most of the West became focused on transport via car.

The popularity of sport increased considerably—both as an activity for all, and as entertainment, particularly on television.

Science advanced dramatically during the century. There were new and radical developments in the physical, life and human sciences, building on the progress made in the 19th century.[14]Big Science flourished, especially after the Second World War, as funding for science increased. Mathematics became ever more specialized and abstract.

A much better understanding of the evolution of the universe was achieved, its age (about 13.75 billion years) was determined, and the Big Bang theory on its origin was proposed and generally accepted.

The age of the solar system, including Earth, was determined, and it turned out to be much older than believed earlier: more than 4 billion years, rather than the 20 million years suggested by Lord Kelvin in 1862.[15]

The planets of the solar system and their moons were closely observed via numerous space probes. Pluto was discovered in 1930 on the edge of the solar system, although in the early 21st century, it was reclassified as a plutoid instead of a planet proper, leaving eight planets.

No trace of life was discovered on any of the other planets in our solar system (or elsewhere in the universe), although it remained undetermined whether some forms of primitive life might exist, or might have existed, somewhere. Extrasolar planets were observed for the first time.

One of the prominent traits of the 20th century was the dramatic growth of technology. Organized research and practice of science led to advancement in the fields of communication, engineering, travel, medicine, and war.

The number and types of home appliances increased dramatically due to advancements in technology, electricity availability, and increases in wealth and leisure time. Such basic appliances as washing machines, clothes dryers, furnaces, exercise machines, refrigerators, freezers, electric stoves, and vacuum cleaners all became popular from the 1920s through the 1950s. The microwave oven became popular during the 1980s. Radios were popularized as a form of entertainment during the 1920s, which extended to television during the 1950s. Cable television spread rapidly during the 1980s. Personal computers began to enter the home during the 1970s–1980s as well. The age of the portable music player grew during the 1960s with the development of 8-track and cassette tapes, which slowly began to replace record players. These were in turn replaced by the CD during the late 1980s and 1990s. The proliferation of the Internet in the mid-to-late 1990s made digital distribution of music (mp3s) possible. VCRs were popularized in the 1970s, but by the end of the 20th century, DVDs were beginning to replace them.

The first airplane was flown in 1903. With the engineering of the faster jet engine in the 1940s, mass air travel became commercially viable.

The assembly line made mass production of the automobile viable. By the end of the 20th century, billions of people had automobiles for personal transportation. The combination of the automobile, motor boats and air travel allowed for unprecedented personal mobility. In western nations, motor vehicle accidents became the greatest cause of death for young people. However, expansion of divided highways reduced the death rate.

The triode tube, transistor and integrated circuit revolutionized computers, leading to the proliferation of the personal computer in the 1980s and cell phones and the public-use Internet in the 1990s.

In addition to human spaceflight, unmanned space probes became a practical and relatively inexpensive form of exploration. The first orbiting space probe, Sputnik 1, was launched by the Soviet Union in 1957. Over time, a massive system of artificial satellites was placed into orbit around Earth. These satellites greatly advanced navigation, communications, military intelligence, geology, climate, and numerous other fields. Also, by the end of the 20th century, unmanned probes had visited the Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and various asteroids and comets. The Hubble Space Telescope, launched in 1990, greatly expanded our understanding of the Universe and brought brilliant images to TV and computer screens around the world.

Cocaine/crack and heroin were found to be dangerous addictive drugs, and their wide usage had been outlawed; mind-altering drugs such as LSD and MDMA were discovered and later outlawed. In many countries, a war on drugs caused prices to soar 10x-20x higher, leading to profitable black marketdrugdealing, and to prison inmate sentences being 80% related to drug use by the 1990s.

Contraceptive drugs were developed, which reduced population growth rates in industrialized countries.

The development of medical insulin during the 1920s helped raise the life expectancy of diabetics to three times of what it had been earlier.

Vaccines, hygiene and clean water improved health and decreased mortality rates, especially among infants and the young.

Sedentary lifestyles, due to labor-saving devices and technology, contributed to an "epidemic" of obesity, at first in the rich countries, but by the end of the 20th century spreading to the developing world.

Widespread use of petroleum in industry—both as a chemical precursor to plastics and as a fuel for the automobile and airplane—led to the geopolitical importance of petroleum resources. The Middle East, home to many of the world's oil deposits, became a center of geopolitical and military tension throughout the latter half of the century. (For example, oil was a factor in Japan's decision to go to war against the United States in 1941, and the oil cartel, OPEC, used an oil embargo of sorts in the wake of the Yom Kippur War in the 1970s).

Communications and information technology, transportation technology, and medical advances had radically altered daily lives. Europe appeared to be at a sustainable peace for the first time in recorded history. The people of the Indian subcontinent, a sixth of the world population at the end of the 20th century, had attained an indigenous independence for the first time in centuries. China, an ancient nation comprising a fifth of the world population, was finally open to the world in a new and powerful synthesis of west and east, creating a new state after the near-complete destruction of the old cultural order. With the end of colonialism and the Cold War, nearly a billion people in Africa were left in new nation states after centuries of foreign domination.

The world was undergoing its second major period of globalization; the first, which started in the 18th century, having been terminated by World War I. Since the US was in a dominant position, a major part of the process was Americanization. The influence of China and India was also rising, as the world's largest populations were rapidly integrating with the world economy.

Terrorism, dictatorship, and the spread of nuclear weapons were some issues requiring attention. The world was still blighted by small-scale wars and other violent conflicts, fueled by competition over resources and by ethnic conflicts. Despots such as Kim Jong-il of North Korea continued to lead their nations toward the development of nuclear weapons.

Disease threatened to destabilize many regions of the world. New viruses such as SARS and West Nile continued to spread. Malaria and other diseases affected large populations. Millions were infected with HIV, the virus which causes AIDS. The virus was becoming an epidemic in southern Africa.

The geographic distribution of surface warming during the 21st century calculated by the HadCM3 climate model if a business as usual scenario is assumed for economic growth and greenhouse gas emissions. In this figure, the globally averaged warming corresponds to 3.0 °C (5.4 °F).

Based on research done by climate scientists, the majority of the scientific community consider that in the long term environmental problems may threaten the planet's habitability.[18] One argument is that of global warming occurring, and that it may be due (at least partially) to human-caused emission of greenhouse gases, particularly carbon dioxide produced by the burning of fossil fuels.[19] This prompted many nations to negotiate and sign the Kyoto treaty, which set mandatory limits on carbon dioxide emissions.

^"Total radiative forcing is positive, and has led to an uptake of energy by the climate system. The largest contribution to total radiative forcing is caused by the increase in the atmospheric concentration of CO2 since 1750." (p 11) "From 1750 to 2011, CO2 emissions from fossil fuel combustion and cement production have released 375 [345 to 405] GtC to the atmosphere, while deforestation and other land use change are estimated to have released 180 [100 to 260] GtC." (p 10), IPCC, Climate Change 2013: The Physical Science Basis - Summary for Policymakers, Observed Changes in the Climate System, p. 10&11, in IPCC AR5 WG1 2013.